This invention relates to catalysts, a method of producing the catalysts and a process for using the catalysts. In another aspect the invention relates to a particularly effective olefin polymerization catalyst and process.
In the production of polyolefins, such as for example polyethylene, polypropylene, ethylene-butene copolymers etc., an important aspect of the various processes and catalysts used to produce such polymers is the productivity. By productivity is meant the amount or yield of solid polymer that is obtained by employing a given quantity of catalyst. If the productivity is high enough then the amount of catalyst residues contained in the polymer is low enough that the presence of the catalyst residues does not significantly affect the properties of the polymer and the polymer does not require additional processing to remove the catalyst residues. As those skilled in the art are aware, removal of catalyst residues from polymer is an expensive process and it is very desirable to employ a catalyst which provides sufficient productivity so that catalyst residue removal is not necessary.
In addition to productivity of a catalyst, another important aspect of a process and a catalyst is the properties of the polymer particles. It is desirable to produce polymer particles which are characterized by strength, uniformity of size, and relatively low fines. Although polymer fluff having relatively high percentages of fines can be handled with plant modifications, a polymer of high productivity and low fines is highly desirable.
U.S. Pat. No. 4,363,746 discloses catalysts which are much more active than many of the polymerization catalysts previously existing. One such catalyst is prepared by reacting a metal halide, such as magnesium dichloride, and a transition metal compound, such as a titanium tetraalkoxide, to form a first catalyst component then reacting a solution of that first catalyst component with an organometallic reducing agent to obtain a solid, and then reacting the solid with a halide ion exchanging source such as titanium tetrachloride. Although such catalysts are extremely active, experience has shown that when they are used in commercial-scale loop reactors the level of fines is often greater than might be desired.
U.S. Pat. No. 4,326,988 discloses that by incorporating prepolymer on such catalysts one can reduce the level of polymer fines in the product without seriously affecting the productivity of the catalyst. The approach of employing prepolymer, however, adds the requirement of additional steps and reactants in the catalyst preparation and thus adds to the expense of producing the catalyst.
An object of the present invention is to produce modified catalysts of the type disclosed in U.S. Pat. No. 4,363,746 which provide excellent productivities and low levels of polymer fines without having to resort to the use of prepolymer.